A fracturing fluid is a fluid that is pumped into a hydrocarbon-bearing reservoir in a geological formation under high pressure to open fractures in the formation, thereby to facilitate the flow of hydrocarbons from the formation. Fracturing fluids are preferably viscous, so as to be able to carry proppants into the fractures that are forced open in the formation.
Conventional fracturing fluids contain high molecular weight polysaccharides based polymers, as gelling agents. These polymers are associated with build-up of filter cake on the fracture face. If the filter cake is not completely removed, it will impede flow of reservoir fluid and hence reducing the effectiveness of the fracture. Control and limiting of residual filter cake becomes extremely important when dealing with problematic formations. Alternative to the conventional polymeric system is the novel development of the present invention.
The present invention is a non-polymeric visco-elastic system. The system is based on surfactant chemistry. Although surfactant based systems have been employed in gravel packing operations since the early 1980s (SPE 17168), further development and refinement of surfactant chemistry has yielded surfactant based fracturing fluids. Some of these techniques are discussed and revealed in Canadian Patent No. 1,298,697 and U.S. Pat. No. 5,964,295.
The advantage of a surfactant based fracturing fluid over a polymeric gel based fluid is that micelle formation in surfactant fluids is virtually instantaneous, but does not alter the actual chemical composition of the fluid. That is, once a critical concentration of surfactant molecules is reached, they will aggregate as micelles, thereby increasing the viscosity of the fluid, but without changing the chemical concentration. Therefore, no chemical initiator is required, and viscosity increase occurs uniformly throughout the fluid.
The key to the present invention therefore is the novel use of amphoteric glycinate surfactant, as an additive. In acidic conditions, the glycinate exhibits cationic properties. When the glycinate is combined in proper ratio with anionic salt such as Sodium Xylene Sulfonate in a neutral to acidic environment of an aqueous stimulation fluid (water or acid) it is believed to form highly structured three dimensional micelles. The interference/interaction of the micelles imparts the desired viscoelastic properties to base stimulation fluid. The required cationic activity of the glycinate is ensured by utilization of an organic acid in the formulation of the additive. The purpose of a low molecular alcohol used in the system is to serve as a dispersability agent for making the system field friendly.
The viscoelastic properties imparted to stimulation fluid are controlled by three mechanisms:                1. By varying total additive added to the stimulation fluid (0.1-5.0%) by volume;        2. By controlling the ratio between the organic/inorganic salt and the glycinate (0.15-0.6% of glycinate); and        3. By controlling or adjusting the pH of the fracturing fluid system.        
Another novel use of amphoteric surfactant is the utilization of change in its ionic properties with pH to control the break mechanism of the gel. As the pH of the system is increased above 6.5 the ionic properties of the glycinate change from cationic to anionic. This change de-stabilizes the micellar structure, hence, resulting in the break of the gel, allowing for easy post frac cleanup. In the earlier technologies, to attain a break the system had to encounter formation fluids (oil and/or water) in order to de-stabilize the gel structure. Simple adjustment in pH did not break the gel in the earlier inventions. This limited the use of the system to those wells that contained oil or those that produced condensate. In the present invention the pH of the system can be increased easily by utilization of alkaline compounds such as carbonates, oxides, amines and etc.
As the temperature of the system increases the interaction between the ion weakens, resulting in decrease in stability of the micelles. The upper limit of the gel appears to be around 65° C. The upper temperature range may be further increased by utilization with alternative salts or using a surfactant with different length of the alkyl group.
In a broad aspect, then, the present invention relates to a fracturing fluid comprising:                (i) a surfactant having the general formula        
                 where R1-R2 are each an aliphatic group of C1-C4, branched or straight chained, saturated or unsaturated, R3 is a group of C12-C22, branched, straight chained or cyclic, saturated or unsaturated;        (ii) a water soluble or dispersible anionic organic salt;        (iii) an acid; and        (iv) a low molecular weight organic solvent.        
The attached drawing is a graph of viscosity % against pH.
It will be observed, then, that the present invention is a four component system, although the four components are typically combined into a single component prior to addition to the fluid being viscosified. The primary component, the surfactant, is preferably a dihydroxyethyl tallow glycinate having the structure:

The second component, the salt is preferably sodium xylene sulfonate. However, other similar salts may be used, such as potassium, zinc, ammonium, magnesium (etc.) xylene or toluene sulfonate. Inorganic salts such as sodium chloride or potassium chloride (KCL) can also be used.
As to the acid component, any organic or mineral acid may be used to lower the pH below 6.5. Acids that have been found to be useful include formic acid, citric acid, hydrochloric acid, and so on. The preferred acid is acetic acid because it is universally available, low cost, and safe to handle.
The final component, the alcohol solvent is used to modify the viscosity of the solvent, usually water, by altering its polarity, which will result in reduced viscosity of the miscellar formations.
In an aqueous fluid, the surfactant composition of the present invention is added in a concentration of from about 0.1% wt to about 5.0% wt. The actual composition in a preferred composition will be, for example:
Glycinate0.65Organic salt0.20Acid0.025Solvent0.125
Some variations, up to about 50% per component, are possible.
Unfoamed, the upper temperature limit of the present invention is about 65° C. However, formulation of the present invention is also compatible with CO2, N2 and CO2, N2, air and low molecular weight hydrocarbon gas foams, over a temperature range of 5° C.-80° C.
The addition of Nitrogen (N2) or Carbon Dioxide (CO2) as either a liquid or gas can dramatically improve the flow back characteristics of fracture fluids especially in under pressured reservoirs. Not only is the amount of fluid required for the job reduced, the hydrostatic head of the well bore fluid is lowered and the well can flow on its own. High pumping pressures will compress Nitrogen bubbles during placement of the stimulation treatment only to expand when pressures are bled off and the treatment is fled back. The fluid contains 10-200 standard cubic metres of Nitrogen or Carbon Dioxide per cubic metre of fluid.
Liquid or gaseous CO2 can also be used to act as the energizing phase, although it is pumped as a liquid and returns to surfaces as a gas. The thermodynamic properties of liquid/gaseous CO2 make it a unique fluid for fracturing. The fluid is pumped at low temperatures of typically −5 to −25° C. and it remains as a critical fluid (single phase) while the fracture is created and the proppant placed. Liquid CO2 that leaks off from the fracture quickly rises to reservoir temperature, increasing its specific volume and becoming more like a gas. When the pressure is dropped and the well is flowed the single phase fluid returns to surface as CO2 vapor.
Treatment design for N2 energized fluid systems typically utilizes N2 pumped at ratios of 40-60 scm/m3, which means standard cubic meters of Nitrogen gas per cubic meter of fluid (water). This is normally sufficient to allow the well to flow unassisted with reservoir pressure gradients of 6 kPa/m or higher. For energized CO2 systems, liquid CO2 is added at equivalent ratios, once the conversion from liquid to gas is made (543 scm/m3). Combined down hole fracturing pump rates are approximately 3.5 m3/min.
Foam Fracturing
When liquid CO2 is used as the energizing medium, the resulting mixture is often referred to as an emulsion, since both phases are liquids. However, herein the term foam is used for describing both N2 or Liquid or gaseous CO2 foamed systems. A foam fracture treatment consists of nitrogen (typically 75%) dispersed as small bubbles throughout a continuous liquid phase. In traditional foams, the liquid phase contains surfactants and gellants to prevent coalescence and resulting phase separation. Foam quality (N2 or Liquid CO2) should range from 52-95% (ratio of gas volume to foam volume). Above 95% the mixture can be defined as a mist with the gas becoming the continuous phase. Below 52% there is no bubble to bubble interference and therefore a stable foam does not exist. Above 52% the gas concentration is high enough for the bubbles to interfere and deform, thereby imparting resistance to shear and increasing the viscosity of the fluid system.
Unlike conventional foams that utilize long-chained polymers to viscosify the external phase, the present invention, foamed, uses a combination of two surfactants to impart viscosity to the water phase. The chemical structure that is formed produces a three-dimensional-matrix of molecules that interfere with each other and raise the apparent viscosity of the base fluid system. Combined with either N2 or Liquid CO2, foam viscosities are generated as a function of foam quality and the fluid system develops all of the properties desired for a fracture fluid system. The use of surfactants also produces a fluid with strong foaming tendencies that aids the return of fluids from the reservoir when foamed with the producing hydrocarbon gas.
Field Trials
With depths ranging from 450-1600 m, trial stimulation treatments have focussed on Cretaceous sandstones that range in reservoir pressure from 2-7 kPa/m. Fracture fluid cleanup has been reported as superior to conventional foams utilizing polymer base gels. Some of the stimulated reservoirs include Glauconite, Dinosaur Park, Bad Heart, Viking, Belly River, Colony, Sunburst, Bluesky, Dunvegan, Mannville, and Bow Island. Long term production data is not available at this time, but comments from trial operations are very positive on fast clean-up and initial production. Also, these reservoirs represent trial stimulation work to date and do not suggest any limitations on the potential for usage on other gas-bearing reservoirs.
Typical Treatments
LOCATIONFORMA-AVERAGEPROPPANTFRACTIONRATE AVE.VOL. MAXJOB TYPEGRADIENTDEPTHPRESSURECONC.N2 Energized69-22W5Bad Heart2.8 m3/min25 tonneCWS 40022 kPa/m625 m9.5 Mpa1070 kg/m3CO2 Energized17-24W4Ostracod2.8 m3/min10 tonneCWS 40018 kPa/m1668 m27.0 Mpa1012 kg/m3N2 Foamed33-5W4Colony3.4 m3/min15 tonneCWS 40022 kPa/m858 m18.3 Mpa743 kg/m3CO2 Foamed20-26W4Belly River4.2 m3/min5 tonneCWS 40017 kPa/m969 m12.0 Mpa560 kg/m3Note:CWA 400 is the fluid of the present invention.Post Fracture Flow Rate Comparison
MEDICINEL. MILKU. MILKJOBLOCA-HATRIVERRIVERTYPETIONFLOW RATEFLOW RATEFLOW RATEN221-17W411,680 m3/day2,650 m3/day1,200 m3/dayEnergizedBorateN221-17W410,860 m3/day4,610 m3/day2,700 m3/dayenergizedCWS 400Lab Test Data
The following examples are used to illustrate viscosifying properties of the fluid of the present invention.